1. Field of the Invention
This invention pertains to a flexographic printing element and a method for engraving the printing element by laser radiation to form a relief so that the element is useful as a flexographic printing plate.
2. Description of Related Art
Printing plates are well known for use in flexographic printing, particularly on surfaces which are corrugated or smooth, such as packaging materials, e.g., cardboard, plastic films, etc. One type of flexographic printing plate is made from vulcanized rubber. Commercial rubbers can be natural or synthetic, such as ethylenepropylenediene monomer (EPDM) elastomers.
Lasers can develop sufficient power densities to ablate certain materials. Lasers such as high-power carbon dioxide lasers can ablate many materials such as wood, plastic and rubber and even metals and ceramics. Once the output from a laser is focused at a particular point on a substrate with a suitable power density, it is possible to remove material in depth to create a relief. Areas not struck by the laser beam are not removed. Thus, the use of the laser offers the potential of producing very intricate engravings in the proper material.
Recently, it has been possible to laser engrave a rubber element directly to provide the desired relief surface necessary for flexographic printing. Laser engraving has provided a wide variety of opportunities for rubber printing plates. Highly concentrated and controllable energy lasers can engrave very fine details in rubber. The relief of the printing plate can be varied in many ways. Very steep as well as gently decreasing relief slopes can be engraved so as to influence the dot gain of such plates. EPDM rubber can be laser engraved to form flexographic printing plates.
U.S. Pat. No. 3,459,733 issued to Caddell describes a method for producing polymer printing plates. The printing plate is made by exposing a layer of the polymeric material to a controlled laser beam of sufficient intensity to ablate the polymer and form depressions on the surface.
U.S. Pat. No. 5,798,202 and U.S. Pat. No. 5,804,353 disclose processes for making a flexographic printing plate by laser engraving a reinforced elastomeric layer on a flexible support. The process disclosed in U.S. Pat. No. 5,798,202 involves reinforcing and laser engraving a single-layer flexographic printing element comprised of a reinforced elastomeric layer on a flexible support. The elastomeric layer is reinforced mechanically, or thermochemically, or photochemically or combinations thereof. Mechanical reinforcement is provided by incorporating reinforcing agents, such as finely divided particulate material, into the elastomeric layer. Photochemical reinforcement is accomplished by incorporating photohardenable materials into the elastomeric layer and exposing the layer to actinic radiation. Photohardenable materials include photocrosslinkable and photopolymerizable systems having a photoinitiator or photoinitiator system.
The process disclosed in U.S. Pat. No. 5,804,353 is similar to U.S. Pat. No. 5,798,202 except that the process involves reinforcing and laser engraving a multilayer flexographic printing element comprised of a reinforced elastomeric top layer, and an intermediate elastomeric layer on a flexible support. The elastomeric layer is reinforced mechanically, or thermochemically, or photochemically or combinations thereof. Mechanical and photochemical reinforcement is accomplished in the same manner as described by U.S. Pat. No. 5,798,202. The intermediate elastomeric layer may be reinforced as well.
A problem associated with the elastomeric elements which are reinforced both mechanically and photochemically is that laser engraving does not efficiently remove the elastomeric material to provide desired relief quality, and ultimately printing quality. It is desirable to use an additive in the elastomeric layer which is sensitive to infrared light in order to enhance the engraving efficiency of the element. Photochemically reinforcing the element provides the desired properties for engraving as well as in its end-use as a printing plate. However, the presence of the additive as particulate or other absorbing material tends to reduce the penetration of the ultraviolet radiation required to photochemically reinforce the element. If the elastomeric layer is insufficiently cured during photochemical reinforcement, the laser radiation cannot effectively remove the material and poor relief quality of the engraved area results. Further, the debris resulting from laser engraving tends to be tacky and is difficult to completely remove from the element. Additionally, if the element is not sufficiently photochemically reinforced the required end-use properties as a printing plate are not achieved. These problems tend to be exacerbated with increasing concentration of the additive that enhances engraving efficacy.
It is an object of the present invention to overcome the disadvantages of the prior art and to provide an elastomeric element which can be laser engraved to form a relief surface suitable for use as a flexographic printing plate.
In accordance with this invention there is provided a laser-engravable flexographic printing element comprising a support; and at least one laser engravable reinforced elastomeric layer of an elastomeric composition comprising an elastomeric binder, at least one monomer, a photoinitiator system that decreases in UV absorbance as polymerization proceeds, and at least one additive containing Sixe2x80x94O functionality or Pxe2x80x94O functionality which absorbs infrared radiation, preferably at 9 to 12 micrometers.
In accordance with another aspect of this invention there is provided a method for forming a flexographic printing plate from the above flexographic printing element comprising reinforcing the elastomeric layer of the elastomeric composition by blanket exposure to ultraviolet radiation and exposing the reinforced element imagewise with infrared laser radiation at 9 to 12 micrometers to engrave the elastomeric layer.
The term xe2x80x9claser engravablexe2x80x9d as used herein refers to materials capable of absorbing laser radiation such that those areas of the materials which are exposed to a laser beam of sufficient intensity become physically detached with sufficient resolution and relief depth to be suitable for flexographic applications. By xe2x80x9cphysically detachedxe2x80x9d, it is meant that the material so exposed is either removed or is capable of being removed by any mechanical means such as by vacuum cleaning or washing or by directing a stream of gas across the surface to remove the loosened particles.
The present invention involves a flexographic printing element which includes on a support at least one reinforced elastomeric layer of an elastomeric composition comprising an elastomeric binder, at least one monomer, a photoinitiator system that decreases in ultraviolet (UV) absorbance as polymerization proceeds, and an additive sensitive to infrared radiation. The flexographic printing element is engravable with laser radiation to physically detach the reinforced elastomeric layer and form relief depth in the areas irradiated with the laser. The elastomeric layer formed from the elastomeric composition is reinforced both by photochemical means in that the composition is photosensitive and hardens upon exposure to actinic radiation and by mechanical means in that the composition includes at least one additive.
Suitable elastomeric binders should be chosen so that the resulting element can be laser engraved as discussed below. In addition, the resulting plate should have the characteristics associated with flexographic printing. These characteristics include flexibility, resilience, Shore A hardness, ink compatibility, ozone resistance, durability and resolution. It is also preferred, but not essential, that such materials do not incorporate halogens or heteroatoms such as sulfur so as to avoid any toxic gases being emitted during the laser engraving process. Thus, either a single elastomeric material or a combination of materials can be used so long as the characteristics desired for flexography are obtained. The binder is present from 40 to 75%, and preferably at least 45% to 55%, by weight of the elastomeric composition.
Examples of elastomeric materials are described in Plastic Technology Handbook, Chandler et al., Ed., (1987). In many cases it maybe desirable to use thermoplastic elastomeric materials to formulate the elastomeric layer. When a thermoplastic elastomeric layer is reinforced photochemically, the layer remains elastomeric but is no longer thermoplastic after such reinforcement. This includes, but is not limited to, elastomeric materials such as copolymers of butadiene and styrene, copolymers of isoprene and styrene, styrene-diene-styrene triblock copolymers, etc. as described in U.S. Pat. Nos. 4,323,636, 4,430,417, and 4,045,231. These triblock copolymers can be divided into three basic types of polymers: polystyrene-polybutadiene-polystyrene (SBS); polystyrene-isoprene-polystyrene (SIS); or polystyrene-poly(ethylenebutylene)-polystyrene (SEBS).
In addition to the elastomeric binder, a second binder may optionally be present in the elastomeric layer. A suitable second binder includes non-crosslinked polybutadiene and polyisoprene; nitrile elastomers; polyisobutylene and other butyl elastomers; polyalkyleneoxides; polyphosphazenes; elastomeric polymers and copolymers of acrylates and methacrylate; elastomeric polyurethanes and polyesters; elastomeric polymers and copolymers of olefins such as ethylene-propylene copolymers and non-crosslinked EPDM; elastomeric copolymers of vinyl acetate and its partially hydrogenated derivatives. The term elastomers, as used herein, also encompasses core-shell microgels and blends of microgels and preformed macromolecular polymers.
The elastomeric composition may also be considered a photopolymerizable composition since the elastomeric composition is photosensitive and reinforced, that is, hardened by polymerization induced by exposure to radiation, preferably ultraviolet radiation. As used herein, the term xe2x80x9cphotopolymerizablexe2x80x9d is intended to encompass systems which are photopolymerizable, photocrosslinkable, or both. The elastomeric or photopolymerizable composition includes an elastomeric binder, at least one monomer, and an initiator having sensitivity to actinic radiation. In most cases, the initiator will be sensitive to visible or ultraviolet radiation. Any photopolymerizable compositions which are suitable for the formation of flexographic printing plates can be used for the present invention. Examples of suitable compositions have been disclosed, for example, in Chen et al., U.S. Pat. No. 4,323,637; Grxc3xctetzmacher et al., U.S. Pat. No. 4,427,749; Feinberg et al., U.S. Pat. No. 4,894,315; and Martens U.S. Pat. No. 5,015,556.
The elastomeric composition (as well as the photopolymerizable composition) contains a single monomer or mixture of monomers which must be compatible with the binder to the extent that a clear, non-cloudy photosensitive layer is produced. Monomers that can be used in the elastomeric composition are well known in the art and include but are not limited to addition-polymerization ethylenically unsaturated compounds having relatively low molecular weights (generally less than about 30,000). Preferably, the monomers have a relatively low molecular weight less than about 5000. Examples of suitable monomers include, but are not limited to, t-butyl acrylate, lauryl acrylate, the acrylate and methacrylate mono- and poly-esters of alcohols and polyols such as alkanols; alkylene glycols; trimethylol propane; ethoxylated trimethylol propane; pentaerythritol; dipentaerylitol; and the like. Other examples of suitable monomers include acrylate and methacrylate derivatives of isocyanates, esters, epoxides and the like. Further examples of monomers can be found in Chen U.S. Pat. No. 4,323,636; Fryd et al., U.S. Pat. No. 4,753,865; Fryd et al., U.S. Pat. No. 4,726,877 and Feinberg et al., U.S. Pat. No. 4,894,315. The monomer is present in at least an amount of 5%, preferably 10%, by weight of the elastomeric composition.
The elastomeric composition includes a photoinitiator which decreases absorbance of ultraviolet (UV) radiation as polymerization proceeds. Photoinitiators which decrease absorbance of UV radiation as polymerization proceeds are known as bleachable photoinitiators which allow UV radiation to continue to expose the photopolymerizable material as crosslinking occurs. Bleachable photoinitiators are different from conventional photoinitators for photopolymerizable materials since conventional initiators continue to absorb actinic radiation after exposure has begun. However, once bleachable photoinitiators react in the elastomeric composition, the initiator no longer has the capacity to absorb radiation. That is, bleachable photoinitiators allow UV radiation to penetrate deeper into the layer as polymerization proceeds so that the layer cures more rapidly and completely than photopolymerizable layers with conventional initiators given the same exposure. Also bleachable photoinitiators will provide greater crosslinking density within the reinforced elastomeric layer which will improve the engravability, hardness, durability and ink resistance of the printing plate. When an additive is present in the layer of elastomeric composition, the use of bleachable photoinitiators is particularly useful since the additive tends to decrease the crosslinking efficiency of the layer because of absorption or scattering of actinic radiation. Further, other components or ingredients in the elastomeric layer may also absorb or scatter actinic radiation as well, tending to decrease crosslinking efficiency of the layer. Thus, bleachable photoinitiators provide greater depth of cure in the elastomeric layer. The bleachable photoinitiator is sensitive to wavelengths of 250 nm to 500 nm, preferably about 365 nm. Bleachable photoinitiators suitable for use in the elastomeric composition of the present invention are acylphosphine oxides. Acylphosphine oxides which are alkoxyphenyl-substituted are particularly preferred and are disclosed by Leppard et al. in U.S. Pat. No. 5,767,169. Alkoxyphenyl-substituted bis acylphosphine oxides include, but are not limited to, 2,4,6-trimethyl-benzoyl-diphenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)-2,4-diisobutoxyphenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)-2,4-dioctyloxyphenyl phosphine oxide, bis(2,4,6-trimethylbenzoyl)-2,4-diisopropoxyphenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)-2,4-dihexyloxyphenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)-2,4-di-sec-butoxyphenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)-2-methyl-4-methoxyphenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)-2-propoxy-4-methylphenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)-2,4-diisopentyloxyphenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)-2,6-dimethyl-4-butoxyphenylphosphine oxide, bis(2,6-dimethyoxybenzoyl)-2,4-dioctyloxyphenylphosphine oxide, bis(2,6-dimethyoxybenzoyl)-2,4-diisobutoxyphenylphosphine oxide, bis(2,6-dimethyoxybenzoyl)-2-methyl-4-methoxyphenylphosphine oxide, and bis(2,6-diethyoxybenzoyl)-2-propoxy-4-methylphenylphosphine oxide. Most preferred bleachable photoiniators are bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide and 2,4,6-trimethyl-benzoyldiphenylphosphine oxide. The bleachable photoinitiator is present from 0.05 to 3%, preferably 0.10 to 1.0%, by weight of the elastomeric composition.
The rate of photopolymerization of a photopolymerizable layer is directly proportional to the square root of the light intensity that impinges upon it as well as the square root of the concentration of the light absorbing and radical generating species (photoinitiator). As the photoinitiator concentration is increased, the rate of polymerization increases. However, the light penetrating through a given thickness of the sample decreases and, as a result, the rate of polymerization can decrease. Therefore, there is a need to optimize the photoinitiator concentration to maximize the rate of photopolymerization (or curing rate). This optimum depends on the thickness of photopolymerizable layer as well as the absorbance characteristics of the photoinitiator. Typically, such an optimum is found by empirical studies by those skilled in the art. In the case of the bleachable photoinitiators which are included in the elastomeric composition, it also depends on the rate of bleaching of the initiator, that is, how quickly the initiator reacts, and any remnant absorbance of the products of these initiators. Further, for the elastomeric composition of the present invention, the absorbance, reflectance and scattering effects of the additive or other substances that are in the composition also need to be considered. As a result, typically, the optimum initiator concentration is found for a specific composition (e.g., additive concentration) and a specific thickness by empirical studies.
The elastomeric composition can optionally include conventional photoinitiators that can be any single compound or combination of compounds which are sensitive to actinic radiation, generating free radicals which initiate the polymerization of the monomer or monomers without excessive termination. The photoinitiator is generally sensitive to actinic light, e.g., visible or ultraviolet radiation, preferably ultraviolet radiation. Preferably, the photoinitiator should be thermally inactive at and below 185xc2x0 C. Examples of suitable photoinitiators include the substituted and unsubstituted polynuclear quinones. Examples of suitable systems have been disclosed in Grxc3xcetzmacher U.S. Pat. No. 4,460,675 and Feinberg et al., U.S. Pat. No. 4,894,315. Photoinitiators are generally present in amounts from 0.001% to 10.0% based on the weight of the photopolymerizable composition.
The elastomeric composition further includes at least one additive containing silica-oxygen (Sixe2x80x94O bond) functionality or phosphorus-oxygen (Pxe2x80x94O bond) functionality which absorbs infrared radiation, preferably at 9 to 12 micrometers. The engraving sensitivity of the elastomeric layer is increased by the presence of the additive which is sensitive to the wavelength of laser radiation used for engraving, which preferably is 9 and 12 micrometers. Thus, the sensitivity of at least one additive is matched to the wavelength of laser radiation used for engraving which, in the present invention, is typically a CO2 laser. Increased engraving sensitivity of the elastomeric layer, results in increasing the speed of engraving (compared to elastomeric materials without additive) and reducing the tackiness of the debris generated by engraving. The additive may also enhance the mechanical properties of the elastomeric layer, such as tensile strength, stiffness, tear resistance, and abrasion resistance. Suitable additives include particulate inorganic filler materials containing silica-oxygen single bond functionality such as silicas, clays, talcs, mica, and silicates, for example, calcium silicate and zirconium silicate. Suitable additives also include organic materials containing phosphorous-oxygen single bond functionality such as aromatic phosphates, aromatic phosphites and aromatic phosphonates. Additives with the phosphorous-oxygen single bond may be in particulate, liquid, or polymeric form. Silicates, particularly zirconium silicate, and aromatic phosphates are preferred since they provide enhanced laser engraving sensitivity of the elastomeric layer over the other mentioned additives. That is, the preferred additives absorb infrared radiation to such an extent that given the same laser power, the elastomeric material is removed faster during engraving than an elastomeric layer containing other filler. The additive can have particle sizes up to a few micrometers, but preferably is less than 1 micrometer. The additive is present in the elastomeric composition in 1 to 20%, preferably 5 to 20%, and most preferably 10 to 20%, by weight of the elastomeric composition. Other additives may be included in the elastomeric composition provided that the sensitivity is matched to the wavelength of laser radiation used for engraving.
The elastomeric composition can optionally contain a second filler, different from the additive, to further enhance desired properties for the elastomeric layer. A suitable second filler includes carbon black, graphite, titanium dioxide, calcium carbonate, barium sulfate, aluminum, and alumina. The second filler can have particle sizes upto a few micrometers, but preferably is less than 1 micrometer. For carbon black as the filler, the particle size is usually between 200 and 500 Angstroms in diameter. The elastomeric composition can contain other components depending on the final properties desired. Such components include sensitizers, plasticizers, rheology modifiers, thermal polymerization inhibitors, colorants, antioxidants, and antiozonants.
The thickness of the reinforced elastomeric layer can vary over a wide range depending upon the type of printing element desired. The photopolymerizable layer can be from about 17 to 285 mils (0.04 to 0.72 cm) in thickness or greater. It is also contemplated that the flexographic printing element can include one or more reinforced elastomeric layers of the same or different composition than an adjacent elastomeric layer.
The support can be any flexible material which is conventionally used with photosensitive elements used to prepare flexographic printing plates. Examples of suitable support materials include polymeric films such as those formed by addition polymers and linear condensation polymers, transparent foams and fabrics such as fiberglass, and metals such as aluminum and nickel. A preferred support is a polyester film; particularly preferred is polyethylene terephthlate. The support typically has a thickness from 2 to 10 mils (0.0051 to 0.025 cm), with a preferred thickness of 3 to 8 mils (0.0076 to 0.020 cm). The support may be in sheet form or in cylindrical form, such as a sleeve. Flexible sleeves made of polymeric films are preferred as they typically are transparent to ultraviolet radiation and thereby accommodate backflash exposure for building a floor in the cylindrical printing element. The sleeve may be formed from single layer or multiple layers of flexible material provided that the sleeve has the above-described characteristics. Multiple layered sleeves may include an adhesive layer or tape between the layers of flexible material. A preferred multiple layered sleeve is disclosed in U.S. Pat. No. 5,301,610. The sleeve typically has a wall thickness from 10 to 80 mils (0.025 to 0.203 cm) or more. Other examples of supports suitable for use in the element are disclosed by Bass et al. in U.S. Pat. No. 3,146,709 and by Hoage et al. in U.S. Pat. No. 4,903,597.
An outer surface of the support may optionally bear a subbing layer of an adhesive material or primer to facilitate the adherence of the photopolymerizable layer to the sleeve. In addition, the outer surface of the sleeve may be flame-treated or electron-treated, e.g., corona treated. The treatment or primer layer is particularly useful when the support is formed of a polymeric film.
A coversheet can optionally be used to prevent contamination or damage to the surface to be laser engraved. The optional coversheet is removed prior to laser engraving. Suitable for use as the coversheet are a thin film of polyester, polycarbonate, fluoropolymers, polystyrene, polyethylene, polypropylene, or other strippable materials. Preferred as a coversheet is polyester. The coversheet can also be subbed with a release layer.
The elastomeric composition can be prepared by employing a variety of techniques which are well known in the art. One method which can be used is to mix the components (that is, binder, monomer, initiator, additive, and other ingredients) in an extruder and then extrude the mixture as a hot melt onto a support. It is preferred that the extruder be used to perform the functions of melting, mixing, deaerating, and filtering the composition. To achieve uniform thickness, the extrusion step can be advantageously coupled with a calendering step in which the hot mixture is calendered between two flat sheets or between one flat sheet and a release roll. Alternately, the material can be extruded/calendered onto a temporary support and later laminated to the desired final support. The elements can also be prepared by compounding the components in a suitable mixing device and then pressing the material into the desired shape in a suitable mold. The material is generally pressed between the support and a coversheet. The molding step can involve pressure and/or heat.
The elastomeric composition is primarily reinforced by the inclusion of the additive. The elastomeric composition is also reinforced photochemically by overall exposure to actinic radiation to effect photohardening in depth prior to laser engraving. The layer of elastomeric composition can be photochermically reinforced during the manufacture of the element or during end-use as part of the formation of the element into a printing plate. The radiation source to effect photohardening of the elastomeric layer should be chosen so that the wavelength emitted matches the sensitive range for the bleachable photoinitator. This ultraviolet radiation source should furnish an effective amount of this radiation. In addition to sunlight, suitable high energy radiation sources include carbon arcs, mercury-vapor arcs, fluorescent tubes, and sub lamps are suitable. Lasers can be used if the intensity is sufficient only to initiate photohardening, and not to ablate material. The exposure time will vary depending upon the intensity and the spectral energy distribution of the radiation, its distance from the elastomeric composition, and the nature and amount of the elastomeric composition. A removable coversheet can be present during the exposure step provided that it is removed after exposure and prior to laser engraving.
In the process of using the flexographic printing element, after photohardening the elastomeric element, the printing element is engraved with laser radiation. Laser engraving involves the absorption of laser radiation, localized heating and removal of material in three dimensions. The laser engraving process of the invention does not involve the use of a mask or stencil. This is because the laser impinges the reinforced elastomeric layer to be engraved at or near its focus spot. Thus the smallest feature that can be engraved is dictated by the laser beam itself. The laser beam and the material to be engraved are in constant motion with respect to each other, so that each minute area of the plate (pixel) is individually addressed by the laser. The image information is fed into this type of system directly from the computer as digital data, rather than via a stencil. Any pattern of a single image or multiple images of the same or different images may be engraved.
Factors to be considered when laser engraving include, but are not limited to, deposition of energy into the depth of the element, thermal dissipation, melting, vaprization, thermally-induced chemical reactions such as oxidation, presence of air-borne material over the surface of the element being engraved, and mechanical ejection of material from the element being engraved. Investigative efforts with respect to engraving of metals and ceramic materials with a focused laser beam have demonstrated that engraving efficiency (the volume of material removed per unit of laser energy) and precision are strongly intertwined with the characteristics of the material to be engraved and the conditions under which laser engraving will occur. Similar complexities come into play when engraving elastomeric materials even though such materials are quite different from metals and ceramic materials.
Laser engravable materials usually exhibit some sort of intensity threshold, below which no material will be removed. Below the threshold it appears that laser energy deposited into the material is dissipated before the vaporization temperature of the material is reached. This threshold can be quite high for metals and ceramic materials. However, with respect to elastomeric materials it can be quite low. Above this threshold, the rate of energy input competes quite well with opposing energy loss mechanisms such as thermal dissipation. The dissipated energy near, though not in, the illuminated area may be sufficient to vaporize to material and, thus, the engraved features become wider and deeper. This effect is more pronounced with materials having low melting temperatures.
Laser engraving can be accomplished by any of various types of infrared lasers emitting infrared radiation in 9 to 12 micrometer wavelength, particularly 10.6 micrometers. The removal of material by the laser is aided by the presence of the additive sensitive to infrared radiation in the reinforced elastomeric layer which absorbs the radiation energy generated by the laser. A laser which is particularly suitable for engraving flexographic printing elements is a carbon dioxide laser which emits at 10.6 micrometer wavelength. Carbon dioxide lasers are commercially available at a reasonable cost. The carbon dioxide laser can operate in continuous-wave and/or pulse mode. Capability of operating the laser in both modes is desirable since at low or moderate radiation intensities, pulse engraving may be less efficient. Energy which might heat, even melt the material, but not vaporize it or otherwise cause it to become physically detached is lost. On the other hand, continuous wave irradiation at low or moderate intensities is accumulated in a given area while the beam scans the vicinity of that area. Thus at low intensities continuous wave mode may be preferred. Pulsed mode may be the preferred mode at high intensities because if a cloud of radiation absorbing material were formed, there would be time for it to dissipate in the time interval between pulses and, thus, it would permit a more efficient delivery of radiation to the solid surface.
Typically, the flexographic printing element is mounted onto an exterior of a rotating drum associated with the laser. The laser is focused to impinge on the element on the drum. As the drum is rotated and translated relative to the laser beam, the element is exposed to the laser beam in a spiral fashion. The laser beam is modulated with image data, resulting in a two dimensional image with relief engraved into the element, that is, a three-dimensional element. Relief depth is the difference between the thickness of the floor and the thickness of the printing layer. Alternately, the laser may move relative to the element on the drum.
The laser engravable flexographic printing elements described herein can be optionally treated to remove surface tackiness either before or after laser engraving. Suitable treatments which have been used to remove surface tackiness of styrene-diene block copolymers include treatment with bromine or chlorine solutions, and light finishing, i.e., exposure to radiation sources having a wavelength not longer than 300 nm. It should be understood that such treatment does not constitute a photochemical reinforcement of the elastomeric layer.
In addition, these elements can be subjected to post-laser engraving treatments such as overall exposure to actinic radiation. Exposure to actinic radiation is generally intended to complete the chemical hardening process. This is particularly true for the floor and sidewall surfaces which are created by laser engraving.